The present invention relates to the field of additives for fluids such as automatic transmission fluids (ATF), manual transmission fluids (MTF), traction fluids, fluids for continuously variable transmission fluids (CVTs), dual clutch automatic transmission fluids, farm tractor fluids, and engine lubricants, as well as additives for engine oil.
In the automatic transmission marketplace, where there is rapid engineering change driven by the desire to reduce weight and increase transmission torque capacity, there is a desire for automatic transmission fluids that exhibit a high static coefficient of friction for improved clutch holding and launch capacity. Continuously slipping torque converter clutches and launch clutches, for instance, impose exacting friction requirements on automatic transmission fluids (ATFs). The fluid must have a good friction versus sliding speed relationship, or an objectionable phenomenon called shudder will occur in the vehicle. Transmission shudder is a self-excited vibrational state commonly called “stick-slip” or “dynamic frictional vibration” generally occurring in slipping torque converter clutches. The friction characteristics of the fluid and material system, combined with the mechanical design and controls of the transmission, determine the susceptibility of the transmission to shudder. Plotting the measured coefficient of friction (μ) versus sliding speed (V), commonly called a μ-V curve, has been shown to correlate to transmission shudder. Both theory and experiments support the region of positive to slightly negative slope of this μ-V curve to correlate to good anti-shudder performance of transmission fluids. A fluid which allows the vehicle to operate without vibration or shudder is said to have good “anti-shudder” performance. The fluid should maintain those characteristics over its service lifetime. The longevity of the anti-shudder performance in the vehicle is commonly referred to as “anti-shudder durability”. The variable speed friction tester (VSFT) measures the coefficient of friction with respect to sliding speed simulating the speeds, loads, and friction materials found in transmission clutches and correlates to the performance found in actual use. The procedures are well documented in the literature; see for example Society of Automotive Engineers publication #941883.
The combined requirements of high static coefficient of friction and durable positive slope are often incompatible with traditional ATF friction modifier technology which is extremely well described in the patent literature. Many of the commonly used friction modifiers result in a low static coefficient of friction and are not durable enough on positive slope to be of sufficient use.
For manual transmissions, the synchronizer is one of the more important components of any manual gearbox. Increasing performance, reducing shift force (raising shift quality) and minimizing the between-the-gears energy losses are the primary objectives for a new generation of synchronizer systems. Improvements in the capacity of the brass system and the introduction of formed sintered cones are allowing economical re-engineering of existing synchronizer designs into more efficient designs (see Hoerbiger and Co. Engineering Report 32). Synchronizer materials are becoming more diverse to include materials such as carbon. This may include a range of substrates from graphite, or sintered graphite, materials through to carbon fibre woven or paper type materials. The frictional appetites of these systems can vary and hence friction modifiers selection can be critical. The chemistry of manual transmission lubricating oils needs to be reformulated for these designs to be able to maintain adequate friction in the synchronizer system and protect these parts from wear.
Many of the commonly used friction modifiers result in a low static coefficient of friction and do not have sufficient durability to be of practical use. Traditionally, detergents (the overbased metal salts of organic acids, such as sulfonates, phenates or salicylates and the like) have been used in ATF and MTF formulations. Further, conventional gear oils or manual transmission oils typically contain chemical components, such as active sulfur and surface-active amine organophosphates. While excellent as additives to provide extreme pressure lubrication, in the usual amounts these additives alone give rise to too large a decrease in friction, while also inadequately protecting friction surfaces from abrasive or corrosive wear. While such additives bring benefits to the overall formulation, they add to the complexity of the lubricant formulation in addition to adding cost. Many friction modifiers can be of limited solubility when in an additive package and thus it is beneficial for a highly soluble friction modifier, while maintaining or bettering its friction modifying and antiwear benefits.
There are patents, for example, U.S. Pat. No. 5,750,476, Nibert et al., May 12, 1998, where a type of friction modifier technology used to achieve this performance is described.
Additional patent literature describing technology for retaining positive μ/v or anti-shudder characteristics include U.S. Pat. No. 5,858,929, Sumiejski et al., Jan. 12, 1999. These may employ metal detergents and combinations of friction modifiers.
Teqjui et al. in EP 0976813 disclose high synchromesh durability performance and gear protection of a manual transmission gearbox. Metal detergents are presented as a required component as overbased salicylates and a calcium sulfonate is shown in the comparative examples.
U.S. Pat. No. 4,512,903, Schlicht et al., Apr. 23, 1985, discloses amides prepared from mono- or poly-hydroxy-substituted aliphatic monocarboxylic acids and primary or secondary amines, useful as friction reducing agents.
PCT Publication WO04/007652, Adams et al., Jan. 22, 2004, discloses a fluid composition of (a) a friction modifier derived from the reaction of a carboxylic acid with an amino alcohol, the friction modifier containing at least two hydrocarbyl groups, and (b) a dispersant, which provides good friction properties in an automatic transmission.
U.S. Pat. No. 4,886,612, Higaki et al., Dec. 12, 1989, discloses a lubricating oil comprising at least one of various products, which can be various imidazolines or an oxazoline of the structure:
where R2 and R3 each represent CH2OCOR1, CH2OH or H, prepared by the condensation of a carboxylic acid (or a reactive equivalent thereof) with an amino alcohol; for example, the condensation of two moles of isostearic acid with one mole of tris-hydroxymethylaminomethane (THAM).
In addition, there is a desire to produce friction modifying compounds that are environmentally friendly and that can mitigate the effect of co-additives. Many lubricating fluids, such as engine oils and gear oils, contain a complex mixture of additives employed for various purposes. For example, most formulations require detergents to keep the formulation clean, anti-wear additives to protect against component wear, dispersants to provide increased solubility and so on. Some of these additives decompose into materials that may damage engine components and/or exhaust after-treatment devices. In addition, with respect to detergents in particular, the performance of current friction modifiers tends to be highly dependent on the structure of the organic portion of the detergent. Likewise, some co-additives, such as, for example, detergents, can cause long-term wear to mechanical components.
Thus, there is an interest not only in improving the performance of friction modifying compounds, but also making environmentally friendly friction modifiers that can mitigate the effects of co-additives in lubricant formulations.